Aligning and mounting optical devices (such as mirrors, lenses, lasers, fibers, focal plane arrays, etc.) within a high magnification, diffraction-limited optical system requires expensive fabrication processes and ultra-precision mounting techniques. This is primarily because each optical device must be mounted to millionths of an inch accuracy according to the precise requirements of the optical system. Various alignment mechanisms are used to assure exacting registration of the various components of the optical system. In addition, each component must be accurately positioned, in a strain-free condition, with respect to the intended propagation direction of electromagnetic radiation (e.g., light). The accuracy to which optical devices are both supported and positioned influences to a large extent the optical wavefront quality, or precision, of the optical system. Optical misalignments may be induced in a system during assembly, alignment, adjustment, calibration, or operation of the components. Because optical systems are assembled from several unique parts, at each imperfect interface between optical and housing components, certain stresses will be induced by fastening mechanisms and/or bonding processes.
Moreover, optical devices and hardware are typically installed at standard atmospheric temperatures and pressures. Exposure to environments, especially those associated with military applications, can induce thermally generated stresses, into both optics and opto-mechanical mounts, due to thermal expansion differences.
The type of stress induced onto an optical element determines the type of resulting distortion of its optical surface. One of the most optically-degrading stresses is that induced by bending the optic. Mirrors, distorted by bending loads, can especially degrade an optical wavefront because light reflects off of the surface of a mirror. Reflection of light off mirrors behaves according to the law of reflection, i.e., the angle of reflection equals the angle of incidence. Bending a mirror alters its surface profile, thereby perturbing both the incidence and reflection angles, all along the distorted profile. Thus this type of physical alteration of the mirror surface results in a line of sight “angle doubling error”, as well as a complex distortion of the optical wavefront.
Since the profile distortion of a bent mirror is typically not uniform nor symmetrical, in every direction across the surface of the mirror, the wavefront distortion also not symmetrical. Thus, bending a mirror typically creates astigmatism in the optical wavefront. An astigmatic wavefront is generally saddle-shaped, which means that correction of this aberration also requires an optical surface that is not circularly symmetric, which is very difficult to fabricate. Thus, eliminating or minimizing bending in mirrors is crucial to achieving diffraction-limited optical performance, especially in all-reflective optical systems.
One common method of mounting and aligning an optical device (such as a secondary metal mirror of a telescope) involves diamond point machining the interface surfaces of both the optic and its mount. Once aligned, precision-machined kinematic fasteners are typically used to secure the optical device, to a housing or other mounting structure, to minimize inducing bolt-up stresses between the fasteners, the mirror, and the mount. While diamond point machined surfaces are very flat, they are not perfect, and thus when two of them are mated together the resultant interface is even less coplanar, which typically induces some bending into both the mirror and the mount.
Kinematic hardware, such as pairs of swivel washers, may be employed between a fastener and a mirror, and also between the mirror and its mount. In theory, such washer pairs can eliminate bending stresses at mating interfaces through the use of spherical surfaces that “swivel” to adjust for any angular misalignment between the interface features. While this swiveling capability compensates for angular differences at mating interfaces, there is always friction between the mating swivel surfaces. Friction can prevent perfect angular alignment especially as the attachment fastener is torqued and the friction forces increase. Thus any residual misalignment of these washers can couple the fastener preload forces into bending of the mirror and/or its mount. The higher the preload from the kinematic hardware, and the greater the angular misalignment in the swivel washers, combine to induce even greater bending moments into the mirror. Utilizing swivel washers between a mirror and its mount also adds thickness and location tolerance errors between these position-critical parts, which is typically very undesirable.
In addition to the challenge of achieving “stress-free” mounted mirrors, is the daunting task of positioning the optic to within millionths of an inch in each direction/orientation, & retaining stability over varying environments. For these obvious reasons, along with a number of other less obvious reasons, the implementation of such mechanical attachment methods often culminates in a misaligned optical device, resulting in degraded optical performance of the system. Owing to the myriad of opportunities for degraded optics and optical wavefront, the prudent approach is to design an interface that does not compromise the integrity of the optic, regardless of the final mounted configuration.
Previous attempts have relied upon the combination of diamond point machined mount (or carrier) plates, mated with the diamond point machined back side of a metal mirror, and retained together with kinematic hardware. The mount plate is then positioned using ultra-precision alignment equipment, and the mount plate is bonded in place. This approach facilitates reuse of the mirror, in the event of unacceptable optical performance resulting from a failed alignment process, or following exposure to severe environmental conditions. The removal of the kinematic attachment hardware facilitates removal and reuse of the optic, with only the mount plate and support components to which it is bonded, serving as the sacrificial items. However, diamond point machining is expensive. In addition, the kinematic fasteners used to secure the mirror to the mount plate can be complex.
Reference will now be made to the exemplary embodiments illustrated, and specific language will be used herein to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended.